A number of compounds, one end of which is water-soluble and the other end fat-soluble, form onto the interface of a liquid phase and a gas phase an insoluble film having the thickness of one molecule layer. In this film the said amphiphilic compounds are oriented so that the water-soluble end of the compound is in the liquid phase and the fat-soluble end in the gas phase. Irwin Langmuir's original publication (Langmuir, I. (1917) J. Am.Chem. Soc. 39, 1848) describes the principles according to which the orientation of compounds into monomolecular layers is accomplished. Different methods for producing and studying surface films have been widely dealt with in a monograph by George Gaines (Gaines, G. L. , Jr (1966) Insoluble Monolayers at Liquid-Gas Interfaces, interscience Publishers, John Wiley and Sons Inc., New York 1966).
To make monomolecular surface films, the chosen substance, dissolved in a volatile solvent, such as e.g. chloroform or hexane, is spread onto a liquid surface in a trough. The available surface area of the liquid-gas interface is limited by a barrier resting on top of the trough, in contact with the liquid surface, and by means of which the total area of the monomolecular surface film is either decreased or increased. Thus it is possible to regulate the surface tension of the film which is inversely proportional to the surface pressure of the film. The degree of packing, that is the surface density of the molecules of the film, can be measured by using the so called compression isotherm, by determining the surface pressure as a function of the surface area, in order to obtain the surface area/molecule vs. surface pressure. By arranging for a feedback coupling between the surface pressure sensor (usually a Wilhelmy-plate, dunoyu-ring or a Langmuir-barrier) and the motor controlling the movements of the barrier, it is possible to create conditions where the surface pressure of the film, that is the packing density of the molecules, is maintained constant. This is necessary when the film on the surface of the liquid, typically of water, is to be transferred onto a solid substrate to be coated that can be plastic, glass, a silicon wafer, metal or alike. As a result of one transfer, one layer is deposited onto the substrate. This kind of a monomolecular film transferred onto a solid substrate is called a Langmuir-Blodgett film. By repeating the transfer, it is possible to produce multilayer LB-films.
LB-films attract nowadays great interest because they open new technical prospects i.a in the semiconductor industry as resists, in data communication as photoconductors, in laser technique in the SHG formation as well as in the sensor technique. A problem has been that so far there has not been commercially available a so-called continuously operating LB-method and apparatus.
The U.S. Pat. No. 4,093,757 describes an apparatus and a method for depositing a monomolecular layer of an amphiphilic compound onto a substrate. The apparatus consists of two compartments and the compartments are separated by a horizontally rotatable cylinder partly immersed in the liquid and feeding molecules of the amphiphilic compound from one compartment to the other. The continuous deposition and production of the film is performed by adding dropwise, at a predetermined rate, a film compound, dissolved in an organic solvent, into one compartment and continuously transferring the said compound to the other compartment by rotating the cylinder.
The U.S. Pat. Nos. 4,722,856 and 4,783,348 describe an apparatus of another type for continuously producing films, wherein no moving mechanical components are used in contact with the film, but the film is compressed and transported from one region of the trough to another using a continuous medium flow, either by blowing gas above the liquid surface tangentially to the surface or by draining the surface from an upper level to a lower level and continuously adding the film compound to the upper compartment.